Patent application title: System and Method for Height Triangulation Measurement

Abstract:

A method for height triangulation measurement particularly for measuring
the height of an object on a surface, the method includes: a)
illuminating said object from a known angle with a narrow strip of light,
having a large numerical aperture along said light strip and a small
numerical aperture perpendicular to said light strip; b) imaging said
object from a known angle having a large numerical aperture along said
light strip and a small numerical aperture perpendicular to said light
strip, having an image of said object illuminated by said light strip;
and c) calculating the height of said object from the location of said
light strip on said image.

Claims:

1. A method for height triangulation measurement particularly for
measuring the height of an object on a surface comprising:a) illuminating
said object from a known angle with a narrow strip of light, having a
large numerical aperture along said light strip and a small numerical
aperture perpendicular to said light strip;b) imaging said object from a
known angle having a large numerical aperture along said light strip and
a small numerical aperture perpendicular to said light strip, having an
image of said object illuminated by said light strip; andc) calculating
the height of said object from the location of said light strip on said
image.

2. The method for height triangulation measurement of claim 1, wherein
said narrow strip of light is spatially incoherent.

3. The method for height triangulation measurement of claim 1, further
includes the usage of a first aperture stop located in the direction of
said illumination and second aperture stop located in the direction of
said imaging, wherein said aperture stops have a rectangle like shape
passage with a longer axis located parallel to said light strip, said
aperture stops are located in order to define the numerical apertures of
said illumination channel and said imaging channel at least in one axis.

4. The method for height triangulation measurement of claim 2, further
includes the usage of a symmetrical configuration, wherein from both
sides illuminating and imaging, using two beam-splitters each one enables
to reflect said illumination from a different location and to pass said
reflected image direct for imaging.

5. A system for height triangulation measurement particularly for
measuring the height of an object on a surface, said system comprising:an
illuminating means capable to illuminate said object from a known angle
with a narrow strip of light being spatially incoherent, having a large
numerical aperture along the light strip and a small numerical aperture
perpendicular to the light strip;an imaging means capable to image said
object from a known angle, having an image of said light strip on said
object; andcalculating means to calculate the height of said object from
the location of said light strip on said image.

6. The system for height triangulation measurement of claim 5, further
includes a first aperture stop located in the direction of said
illumination and second aperture stop located in the direction of said
imaging, wherein both said aperture stops have a rectangle like shape
passage, said passage is located parallel to said light strip with a
longer axis located parallel to said light strip, said aperture stops are
located in order to define the numerical apertures of said illumination
channel and said imaging channel.

7. A symmetrical configuration system for height triangulation measurement
particularly for measuring the height of an object on a surface, said
system comprising:first and second illuminating and imaging sets, wherein
said first set is illuminating and imaging said object from an opposite
angle of said second set and wherein each of said sets includes:an
illuminating means capable to illuminate with a narrow strip of light
being spatially incoherent, having a large numerical aperture along the
light strip and a small numerical aperture perpendicular to the light
strip;a beam splitter to reflect said illumination to said object and
enables a direct pass of the reflected image from said object or vice
versa;an imaging means capable to image said object, having an image of
said light strip on said object; andan aperture-stop located in the
direction of said illuminating and imaging; andcalculating means to
calculate both heights of said object from the location of the light
strip on the image as measured by said first and second set and combine
the results.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates to optical triangulation systems, with
the purpose to measure height of small objects such as wafer bumps. This
measurement is essential for process control in semiconductors
fabrication.

BACKGROUND OF THE INVENTION

[0002]Solder bumps have become a standard way to connect semiconductor
devices to substrates and substrates to the Printed Circuit Board (PCB).
It is therefore, that the technology is important for two different
industries, Wafers fabrication and PCB manufacturing. Although there is a
lot of similarity between PCB and wafer bumps, there are also differences
in height and diameter of the bumps, which affect the process control. In
general, wafer bumps are substantially smaller than PCB bumps. Typical
numbers are 100 μm bump height in wafers and 300 μm in PCB, but
there can be large variations from these numbers.

[0003]To have a good connection, all bumps (there may be thousands of
bumps on each die) must be uniform in their height within an allowable
tolerance. Therefore, bumps height metrology is an essential part of the
process control in both fabrications.

[0004]The common way to measure bumps in the PCB industry, is to scan over
the area of an object (PCB or Substrate) with laser line triangulation.
Triangulation has a long history as a 3D measurement tool, and it comes
with a large variety of optical configurations. U.S. Pat. No. 3,187,185
(to Milnes and Pitcairn, June 1965) describes an optical triangulation
system in which a narrow strip of light is projected upon a flat object
from a well-defined angle α. A camera, images the object from
another defined angle β. The height of the object can then be
measured from the position of the strip upon the image, provided that
angles α and β are known and the object is considered to be
flat. In a second embodiment, Milnes and Pitcairn also present a
configuration of two line projectors and one camera. This concept is the
basic configuration for most bumps metrology systems today. As a light
source it is common to use a laser, because it provides high intensity at
a very narrow strip and the spatial coherence of the laser guarantees
good definition of the illumination angle. U.S. Pat. No. 5,028,799 to
Chen et al. describes a laser triangulation system for bumps metrology,
employing two lasers and one camera. Two lasers can overcome several
issues including different optical behavior of the solder and the
substrate.

[0005]While laser triangulation provides satisfactory measurement of PCB
solder bumps, it makes an excessive error measuring small Wafer bumps.
This error is known as "shape-error", and it is typical to small bumps or
other features having smaller size than the illumination (the width of
the strip). Although solder bumps are usually larger than 100 μm in
diameter, since they have a ball shape, only a small section at the top
of the ball can be observed. The nature of shape-error is that a feature
will be measured properly only when located at the centerline of the
strip. When the feature is moved forward or backward, the triangulation
height measurement will have an up or down error respectively. While
scanning, both errors will occur (up and down errors) therefore
theoretically, they can be summed up to zero. This implies that an
averaging along the scanning axis is highly recommended to improve
shape-errors. Averaging alone however, can give satisfactory results in
terms of accuracy (1-3 μm typically), only if the bumps have a smooth,
well reflecting surface. In reality, the processes of manufacturing
solder bumps create surface defects, especially if they are manufactured
in a led-free process. These surface defects do not reflect light
completely or partially, therefore they make it impossible to get enough
data to eliminate the shape-errors by averaging. Large defect located at
the bump-top can even prevent the measurement completely.

[0006]The present invention overcomes these disadvantages by using a
spatially incoherent illumination in a unique optical configuration.

[0007]Another known optical-errors is expressed by the fact that usually
different heights results are achieved from different direction of
scanning. Therefore, a common way of measuring is to scan the object
twice, from right to left and then from left to right and averaging the
results. The "symmetrical configuration" of the present invention enables
the averaging in a single scan.

SUMMARY OF THE INVENTION

[0008]The present invention is a method and a system for height
triangulation measurement particularly for measuring the height of an
object on a surface.

[0009]According to the teachings of the present invention there is
provided a method for height triangulation measurement particularly for
measuring the height of an object on a surface, this method comprising:
[0010]a) illuminating the object from a known angle with a narrow strip
of light being spatially incoherent, having a large numerical aperture
along the light strip and a small numerical aperture perpendicular to the
light strip; [0011]b) imaging the object with a large numerical aperture
along the light strip and a small numerical aperture perpendicular to the
light strip; and [0012]c) calculating the height of the object from the
location of the light strip on the image.

[0013]According to further features in the described preferred
embodiments, the method of the present invention is provided, further
includes the usage of a first aperture-stop located in the direction of
the illumination and second aperture-stop located in the direction of the
imaging, wherein the aperture-stops have a rectangle shape wherein the
longer axis is parallel to the light strip and the narrow axis is
perpendicular to the light strip, the aperture-stops are located in order
to define the numerical apertures of the illumination channel and the
imaging channel.

[0014]According to further features in the described preferred
embodiments, the method of the present invention is provided with a usage
of a symmetrical configuration, wherein from both sides illuminating and
imaging, using two beam-splitters each one enables to reflect the
illumination from a perpendicular location and to pass the reflected
image direct for imaging.

[0015]By another aspect of the preset invention it is provided a system
for height triangulation measurement particularly for measuring the
height of an object on a surface, the system comprising of: [0016]an
illuminating means capable to illuminate with a narrow strip of light
being spatially incoherent; [0017]an imaging means capable to image the
object from a known angle, having an image of the light strip on the
object; and [0018]calculating means to calculate the height of the object
from the location of the light strip on the image.

[0019]In a preferred embodiment, the system of the present invention is
provided, further includes a first aperture-stop located in the direction
of the illumination and second aperture-stop located in the direction of
the imaging, wherein both aperture-stops have a rectangle shape with a
longer axis along the light strip and a narrow axis perpendicular to the
light strip, the aperture-stops are located in order to define the
numerical apertures of the illumination channel and the imaging channel.

[0020]By another aspect of the present invention it is provided a
symmetrical configuration system for height triangulation measurement
particularly for measuring the height of an object on a surface. This
system comprising: [0021]first and second illuminating and imaging
sets, wherein the first set is illuminating and imaging the object from
an opposite angle of the second set and wherein each set includes:
[0022]an illuminating means capable to illuminate with a narrow strip of
light being spatially incoherent, having a large numerical aperture along
the light strip and a small numerical aperture perpendicular to the light
strip; [0023]a beam splitter to reflect the illumination to the object
and enables a direct pass of the reflected image from the object or vice
versa; [0024]an imaging means capable to image the object, having an
image of the light strip on the object; and [0025]an aperture-stop
located in the direction of the illuminating and imaging;
[0026]calculating means to calculate both heights of the object from the
location of the light strip on the image as measured by the first and
second set and average the results.

BRIEF DESCRIPTION OF THE FIGURES

[0027]The invention is herein described, by way of example only, with
reference to the accompanying drawings. With specific reference now to
the drawings in detail, it is stressed that the particulars shown are by
way of example and for purposes of illustrative discussion of the
preferred embodiments of the present invention only, and are presented in
the cause of providing what is believed to be the most useful and readily
understood description of the principles and conceptual aspects of the
invention. In this regard, no attempt is made to show structural details
of the invention in more detail than is necessary for a fundamental
understanding of the invention, the description taken with the drawings
making apparent to those skilled in the art how the several forms of the
invention may be embodied in practice.

[0030]FIG. 3 shows the optical setup of the triangulation metrology system
in symmetrical configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031]The main object of the present invention is to provide a
triangulation method and apparatus for metrology of small features such
as wafer bumps and to reduce the sensitivity to surface defects. The
invented apparatus maintains a long depth of focus and good definitions
of the illumination and imaging angles.

[0032]The invention overcomes the limitations of the prior art by
illuminating the object with a narrow strip of light, which is spatially
incoherent and having a large numerical aperture. This illumination
enables information from a larger section over a ball-shape bump and it
reduces the noise related to surface roughness. To maintain a long depth
of focus and to define the exact angles of illumination versus imaging,
the invention introduces a non-circular aperture, which limits the
numerical aperture in one axis.

[0033]Hence, there is provided according to the teaching of the present
invention a method of improving the triangulation metrology over wafer
bumps and other features comprising the steps of illuminating the object
from a well defined angle with a narrow strip of light being spatially
incoherent and having a large numerical aperture along the strip and a
small numerical aperture perpendicular to the strip; imaging the object
from a different well defined angle with imaging system having a large
numerical aperture along the strip of light and small numerical aperture
perpendicular to the strip; and analyzing the height of the object from
the location of the imaged strip on the imaged object.

[0034]Referring now to the drawings, FIG. 1 shows the optical setup of a
triangulation system to measure small ball-shape bumps. A source strip of
light (11) is projected upon the object (12) through an imaging system
(14). A camera (15) receives the rays of light through an optical imaging
system (16) and the height of the object is calculated from the image
using the angles α (17) and β (18). Aperture stops (19) and
(20) define the numerical apertures of the projecting and imaging
channels. The apertures (19a) and (20a) are designed to allow large
numerical aperture along the strip (axes x1 and x2) and to limit the
numerical aperture in the axis perpendicular to the strip (axes y1 and
y2).

[0035]To understand the roll of the apertures (19) and (20) consider FIG.
2 showing the imaging of a single bump (13). As the bump (13) is
illuminated with angle 2φ (22) and imaged by angle 2θ (23),
only a small section 2ζ (21) upon the bump (13) can direct light
from projection to imaging. The angular size of the section ζ (21)
can be calculated through:

2ζ=φ+θ (1)

Where sin(φ) and sin(θ) are the numerical apertures at the
projection and imaging channels.As the height of the bump (13) can be
measured only upon the section 2ζ (21), it is desired to extend
ζ (21) as large as possible, so surface defects will not prevent the
measurement. That implies large numerical apertures on both channels are
required. On the other hand, there are contradicting demands for high
definition of the illumination and imaging angles, which require small
numerical apertures. The range of measurement by the triangulation system
also requires small numerical apertures for long depth of focus.The depth
of focus on the projection channel (DOF1) can be calculated by:

DOF1=δ/sin(φ) (2)

Where δ is the width of the illuminated strip.In the same manner,
the depth of focus on the imaging channel is:

DOF2=δ/sin(θ) (3)

Like said, long depth of focus and large measurement range requires small
numerical apertures.

[0036]Aperture stops (19) and (20) settle the contradicting requirements.
Front views (19a) and (20a) of the apertures in FIG. 1, show that they
have a rectangular shape, so the numerical aperture along axes y1,
y2 (perpendicular to strip) is limited. This configuration allows
long depth of focus in the sense that the strip remains narrow at a long
range of measurement. At the same time, the large numerical aperture
along the strip allows measurement at a large section on the bump (13) to
overcome shape error and issues of surface defects.

[0037]FIG. 3 shows the optical setup of the triangulation metrology system
in symmetrical configuration that includes two sets of illuminating and
imaging, one from the opposite angle of the other. In this configuration
there are two light sources (11a and 11b) illuminating the bumps (13) on
the object (12) from a perpendicular direction, reflected by
beam-splitters (22a & 22b). The beam-splitters (22a & 22b) enable the
reflected image from the bumps (13) to pass to the imaging means (15a &
15b). The bump (13) height is calculated by both sets and usually having
different results that can be averaged.

[0038]Although the invention has been described in conjunction with
specific embodiments thereof, it is evident that many alternatives,
modifications and variations will be apparent to those skilled in the
art, accordingly, it is intended to embrace all such alternatives,
modifications and variations that fall within the spirit and broad scope
of the appended claims.